When performing a live backup in SQL Server, the database engine uses a combination of transaction log management and page-level copying to maintain consistency while allowing continuous operations. The process involves these key components:

-- Example of basic backup command during operations
BACKUP DATABASE [YourDatabase] 
TO DISK = N'C:\Backups\YourDatabase.bak'
WITH NOFORMAT, INIT, NAME = N'YourDatabase-Full Backup',
COMPRESSION, STATS = 10;

The transaction log plays three critical roles during online backups:

• Records all modifications (LSN - Log Sequence Numbers)
• Maintains before-and-after images of changed data
• Serves as a redo/undo mechanism for recovery

The backup contains a point-in-time snapshot representing the database state when the backup began, plus all committed transactions up to when the backup completes. Consider this timeline:

-- Transaction occurring during backup example
BEGIN TRANSACTION;
UPDATE Products SET Price = 19.99 WHERE ProductID = 101;
-- Backup starts here
INSERT INTO Orders VALUES (...);
-- Backup completes here
COMMIT TRANSACTION;

The backup will include either:

1. The entire transaction if it committed before backup completion
2. None of the transaction if it committed after

While online backups don't block operations, they do impact performance:

-- Monitoring backup performance
SELECT 
    command,
    percent_complete,
    estimated_completion_time/1000 as sec_remaining
FROM sys.dm_exec_requests
WHERE command LIKE '%BACKUP%';

For mission-critical systems, consider these patterns:

-- Differential backup example
BACKUP DATABASE [YourDatabase] 
TO DISK = N'C:\Backups\YourDatabase_diff.bak'
WITH DIFFERENTIAL, COMPRESSION;

-- Transaction log backup example
BACKUP LOG [YourDatabase] 
TO DISK = N'C:\Backups\YourDatabase.trn'
WITH COMPRESSION;

Remember that differential and log backups require an initial full backup as their base.

Always validate your backups with restore tests:

-- Test restore to verify backup integrity
RESTORE DATABASE [YourDatabase_Test] 
FROM DISK = N'C:\Backups\YourDatabase.bak'
WITH MOVE 'YourDatabase' TO 'C:\Data\YourDatabase_Test.mdf',
MOVE 'YourDatabase_log' TO 'C:\Data\YourDatabase_Test_log.ldf',
STATS = 5;

SQL Server employs a sophisticated mechanism called VSS (Volume Shadow Copy Service) combined with transaction log management to enable online backups. When you initiate a backup while the database is running, SQL Server essentially takes a point-in-time snapshot of the database files.

The transaction log (LDF file) acts as the synchronization mechanism during online backups:

-- Example showing backup with transaction log inclusion
BACKUP DATABASE [YourDatabase] 
TO DISK = 'C:\Backups\YourDatabase.bak'
WITH INIT, COPY_ONLY;

The log records all modifications during the backup operation. SQL Server uses this to ensure transactional consistency by:

1. Marking the start LSN (Log Sequence Number) when backup begins
2. Capturing all transactions that occur during backup
3. Recording the end LSN when backup completes

When data changes occur during backup:

-- Example transaction during backup
BEGIN TRANSACTION;
UPDATE Products SET Price = Price * 1.1 WHERE CategoryID = 5;
COMMIT TRANSACTION;

SQL Server handles this through:

• Before-image logging: Original data pages are preserved in the backup
• After-image logging: Changes go to transaction log
• Page protection: Modified pages are copied before changes occur

Consider these performance characteristics:

-- Monitor backup performance
SELECT 
    command,
    percent_complete,
    estimated_completion_time
FROM sys.dm_exec_requests
WHERE command LIKE 'BACKUP%';

Key observations from production:

To verify how modifications during backup affect restore:

-- Create test scenario
CREATE DATABASE BackupTest;
GO
USE BackupTest;
CREATE TABLE TestData (ID INT PRIMARY KEY, Value VARCHAR(100));
GO

-- Start backup in one session
-- In another session:
INSERT INTO TestData VALUES (1, 'Before Backup');
-- During backup:
INSERT INTO TestData VALUES (2, 'During Backup');
-- After backup:
INSERT INTO TestData VALUES (3, 'After Backup');

The restored database will contain records 1 and 2, but not 3, proving transaction consistency during the backup window.